Sludge and debris tend to collect at the bottom of bodies of water both in natural ponds and lakes as well as artificial structures such as ponds used to store radioactive waste. It is required at times to remove/retrieve the sludge to recover contamination from the bottom of these bodies of water without having to remove the water.
Certain types of radioactive waste (such as spent MAGNOX reactor fuel) are stored underwater in storage ponds to provide shielding and cooling of the waste, and in this case a sludge can be formed with time due to for example the material suffering corrosion with the formation of magnesium hydroxide. This sludge will typically be radioactive. The sludge build up can cover other plant, equipment and systems contained within the storage pond. Periodically the build up of sludge needs to be removed. If the pond structure was to fail, for example owing to an earthquake, then the water and agitated sludge could escape into the environment.
A currently known method for the removal/retrieval of such sludge is to extract the sludge (using suction) via a pipe protected by a mesh filter typically with 6 mm diameter holes to prevent larger items of debris being sucked into the system. Typical experience with MAGNOX sludge confirms that filter meshes with holes of this size blind very quickly resulting in the loss of desludging efficiency and thereby requiring downtime to remove the system from the storage pond to carry out routine backwashing and cleaning. This potentially may result in the operating personnel receiving higher doses of radiation. It would therefore be desirable to be able to avoid the need remove the head to remove blinding material from the mesh filter.